The present invention relates to resonant inverters for powering AC loads, preferably gas discharge lamps, and for dimming such lamps. The invention also relates to regulated DC/DC converter circuits.
The following references are incorporated by reference: U.S. Pat. Nos. 5,245,253; 4,998,046; 6,246,183; 5,723,953; and 5,719,472; U.S. patent application Ser. No. US2003/0147263 A1; IR Application Notes AN-995A “Electronic Ballast Using the Cost-Saving IR2155X Drivers”; IR Design Tip DT98-1, “Variable frequency Drive Using IR215X self oscillating IC's”; and “A Resonant Inverter for Electronic Ballast Application,” Melvin C. Cosby and R. M. Nelms, IEEE Transactions On Industrial Electronics, vol. 41, no. 4, August 1994.
A gas discharge lamp typically utilizes electronic ballast for converting AC line voltage to high frequency current for powering the lamp. Conventional electronic ballasts include an AC to DC converter and a resonant inverter converting DC voltage to lamp high frequency current. The resonant inverter includes switching transistors generating a high frequency rectangular AC voltage that is applied to a voltage resonant circuit having an inductor and a capacitor in series. The gas discharge lamp is coupled in parallel to the capacitor. For high frequency electronic ballasts, a self-oscillating resonant inverter is a common part that generates AC voltage for starting and AC current for powering the lamp. Self-oscillating resonant inverters utilize a feed back transformer coupled between a resonant tank circuit and gates of the switching transistors to provide a sinusoidal voltage to the gates for sustaining the oscillations. Resonant inverters are also used in DC /DC converters.
The main advantage of resonant inverters is zero voltage switching that permits operation at higher switching frequencies. A typical resonant inverter comprises a half (or full) bridge with power MOSFETs generating high frequency AC to power a resonant load. Three types of resonant loads are common that differ by real load coupling to LC components: series, parallel and series-parallel circuit configurations. In any combination of resonant load components, the control circuit provides MOSFET switching above resonant frequency for efficient and reliable MOSFET operation. When switching above resonant, the input of the resonant load is inductive. When switching below resonant, this input is capacitive and should be avoided. Self-oscillating inverter circuits built as oscillators with a positive feed back automatically provide a stable inductive mode of operation. In such oscillators, switching frequency advances the resonant frequency of the resonant load and tracks any changes in resonant load.
Ballasts with high frequency oscillating inverter standard industrial controllers and self-oscillating half bridges, such as the IR215X and IR53H(D) series from International Rectifier or the L6579 series from ST Microelectronics and others, do not have the drawbacks of self-oscillating resonant inverter circuits. However, the pre-adjusted switching frequency is not sensitive to resonant frequency changes of the resonant load, and is susceptible to noise and variations of integrated circuit (IC) supply voltage Vcc. In view of this, a direct application of these controllers is not likely. Without correction of switching frequency, the MOSFETs could cross conduct and fail when operating below resonant frequency in some steady-state conditions, dimming mode or, at lamp starting. Also, power control with the above ICs is not provided.
One solution for avoiding this problem is described in Application Notes AN 995A “Electronic Ballasts Using the Cost-Saving IR215X Drivers” issued by International Rectifier. This reference recommends a feed back circuit with two anti-parallel power diodes connected in series with the resonant load as zero current detectors. The diodes generate a rectangular AC pulse signal that forces the timing circuit in the IC to switch synchronously with this signal. A feed back signal indicates phasing of current in the resonant load. However, zero current sensing in any portion of the resonant load does not provide the necessary 360° positive feed back angle for phase locked operation above resonant frequency. In addition, when used as a source of synchronization signals, the power diodes add significant power losses to the ballast.
Other prior art IC driven resonant inverters are disclosed in U.S. Pat. Nos. 5,723,953 and 5,719,472. Both patents teach half bridge IC feed back control by changing sinusoidal control signal amplitude. With this approach, phase shifting is forced to depend on the amplitude of the feed back signal and thus the stability of the oscillating system can be pure, especially during transients.
U.S. patent application Ser. No. 2003/0147263 A1 discloses a phase delay control that controls the inverter. This control has a static feed back circuit having an input signal representing the phase of the inductor current which is compared with a signal representing a reference phase. The difference, or error signal, is supplied to a voltage controlled oscillator (VCO) to control inverter frequency and power. This control technique utilizes active components incorporated in a controller for processing pulse signals.
The present applicant's prior application (Ser. No. 10/649,898) discloses a method for controlling a resonant inverter by synchronization of a self-oscillating driver IC. The method utilizes a voltage attenuated and phase shifted feed back sinusoidal signal for loop lock up. Even so, there is still a need for circuitry with a wider range of control and better robustness and phase shift control.
One of the problems of the prior art circuits for internal synchronization of IC driven resonant half bridge inverters is that they require significant phase rotation to get 360° total phase shift of the feed back signal. It is very desirable, for reliable phase lock up and before closing the loop, that the injected feed back signal is generated with a minimum phase difference relative to the external synchronization signal. It is also very desirable for reliable synchronization that the injected signal be sufficiently above the ramp signal in wide range of operating frequencies. It is also very disable to have inverter output power control by a small external DC signal (as when dimming).